Cloned cytotoxic T lymphocytes were maintained with stable cytotoxic phenotype for over size months. By subcloning and selection, variants were obtained which expressed greater than 10X lower levels of cytotoxicity. In order to test the feasibility using molecular genetics approaches to analyze the small differences between cytotoxic and non cytotoxic cells, a preparation of mRNA from the T cell lymphoma EL4 was made, yielding .08 pg undegraded poly A+ RNA/cell. This suggests that it is feasible to use cytotoxic T cell lines as a source of mRNA to create a cDNA library. Rabbit antibodies against rat NK tumor cytoplasmic granules show some inhibition of cloned CTL cytotoxicity; these antibodies detect cytoplasmic granules in the CTL by immunofluorescence.